C++ pass parameter to lambda function

Question

It's the first time I use lambda functions in C++ and I need to pass a value to use inside the body of a lambda function:

in the code above I need to pass int parameter seq_msg:

void do_connect(tcp::resolver::iterator endpoint_iterator, int seq_msg)
{
    boost::asio::async_connect(socket_, endpoint_iterator, [this](boost::system::error_code ec, tcp::resolver::iterator)
    {
        if (!ec)
        {
            send_message(seq_msg);

            do_read_header();
        }           
    });
}

Edgar Rokjān · Accepted Answer · 2017-06-29 13:48:00Z

6

I guess you should capture it by value and change:

[this]

to:

[this, seq_msg]

answered Jun 29, 2017 at 13:48

Edgar Rokjān

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Comments

Jeka · Accepted Answer · 2017-06-29 13:52:55Z

1

You can use bind

void do_connect(tcp::resolver::iterator endpoint_iterator, int seq_msg)
{
    auto cb = [this](boost::system::error_code ec, tcp::resolver::iterator, int seq)
    {
        if (!ec)
        {
            send_message(seq_msg);

            do_read_header();
        }           
    }
    boost::asio::async_connect(socket_, endpoint_iterator, std::bind(cb, std::placeholders::_1, std::placeholders::_2, seq_msg) );
}

Or just capture seq_msg

answered Jun 29, 2017 at 13:52

Jeka

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2 Comments

Neumann Over a year ago

Thank you all. I tested the options and both work. Now, I just need to understand if there is something important difference between them.

Jeka Over a year ago

If you are using lambda as a callback then no doubt that capture one extra parameter is much easier. Bind approach allow you to create function object from other class functions, pointers to function or another function objects.

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